This invention relates to a method and apparatus for correcrting astigmatism in a scanning electron microscope (SEM), or similar electron beam scanning device.
In an apparatus for obtaining a scanning image such as an SEM. etc., astigmatism correction of the electron optical system (principally the electron magnetic lenses), in order to finely focus the electron beam irradiating the specimen being examined, is an extremely important but quite difficult operation. Hitherto, the operator had to correct astigmatism by alternately adjusting the focus and astigmatism correction knobs while observing the scanning image on a Braun tube, an operation requiring considerable skill to obtain effective results.
The main object of the present invention is to provide a method and apparatus for carrying out astigmatism correction automatically, and thereby enhance the operability of the SEM.